It is known to utilize intermediate transfer surfaces in ink jet printing systems, such as the one disclosed in U.S. Pat. No. 5,389,958 entitled IMAGING PROCESS and assigned to the assignee of the present application. This patent discloses an offset ink jet printing system in which an intermediate transfer drum is employed with a print head. A liquid intermediate transfer surface is applied to the transfer drum. Nozzles in the print head eject drops of ink onto the liquid intermediate transfer surface to form an ink image thereon. A final receiving substrate such as paper is then brought into contact with the intermediate transfer surface, and the ink image is transferred to the final receiving substrate. The liquid intermediate transfer surface is cleaned and reapplied prior to the next image being formed on the transfer surface.
Ink jet printing systems that utilize a liquid intermediate transfer surface generally require an applicator to apply the desired amount of liquid onto the intermediate transfer support surface. An exemplary applicator of this type is disclosed in U.S. Pat. No. 08/382,453, entitled INTERMEDIATE TRANSFER SURFACE APPLICATION SYSTEM and assigned to the assignee of the present application. This application discloses an applicator that is housed in a replaceable transfer drum maintenance cassette. The applicator uses a wick assembly as a contact medium to concurrently apply the liquid onto the intermediate transfer support surface and to remove foreign matter from the support surface. Specifically, as the support surface or transfer drum rotates, the wick assembly is moved into stationary contact with the rotating transfer drum. In this manner, relative motion is created between the rotating transfer drum and the stationary wick such that the transfer drum brushes or rubs against the wick. This allows the wick to contact and remove foreign matter and debris from the drum. However, it also allows debris to accumulate at the point of contact between the drum and the wick, which can interfere with the application of liquid to the drum. This applicator assembly also includes a hydrodynamic wiper blade that uniformly meters and distributes the liquid intermediate transfer surface over the transfer drum.
A supply of liquid for the wick is maintained in two separate oil filled bladders adjacent to the applicator assembly. The release of the oil from the oil bladders is actuated by the movement of the wick assembly upwardly along a valve opening track as the wick assembly moves toward the transfer drum support surface. This movement opens a valving system that allows oil to flow from the bladders through oil access cross bores and spool valve bodies and into a channel that contains the wick. From the channel the oil is wicked upwardly to the upper portion of the wick that contacts the transfer drum.
Prior to installation of the drum maintenance cassette in a printer, the wick is dry, the valving system is closed and the oil does not flow from the bladders to the wick. Upon insertion of the cassette into a printer, the valving system is opened as described above and the oil begins flowing to the wick. To allow the wick to become sufficiently saturated with the oil for proper operation, printing is disabled for a predetermined period, designated the "time-to-first-print," after a new cassette is inserted in a printer.
In addition to the "time-to-first-print" delay and accumulation of debris at the wick/drum contact point, the prior art offset ink jet printing systems that utilize a liquid intermediate transfer surface applicator assembly, such as the one described above, have other limitations in their performance and operation. With the stationary wicking contact medium that creates relative motion with the rotating transfer drum, the amount of fluid delivered by the contact medium can be inconsistent and may vary over time. Where an oil is used as the liquid for the intermediate transfer surface, it is especially important to have a simple and reliable, yet relatively inexpensive and manufacturable applicator assembly that does not leak or erratically dispense the oil. Containment of oil in an applicator assembly that may be removed from the printer after actual use has commenced can be a problem. For example, in the '645 patent described above, oil pools in the bottom of the wick channel and may spill from the channel if the cassette is tilted for an extended period after actual use has begun. This is especially true where the wick has been saturated for an extended period and a substantial pool of oil has accumulated in the wick channel.
Furthermore, insufficient control over the distribution and thickness of the liquid intermediate transfer surface has negative effects on printed image quality. Non-uniform film distribution or improper film thickness around the drum results in undesirable image artifacts. Those areas of the surface that have more fluid may be visible on the image as low gloss spots or streaks. If the intermediate transfer surface becomes too thin or is absent, ink can adhere to the drum and not be transferred. This problem becomes even more critical when the final receiving surface for the image is an overhead transparency. In this case, projection of the printed image magnifies areas of non-uniform fluid distribution.
In systems utilizing bladders or other reservoirs to supply liquid to a contact medium, the bladders or reservoirs must be refilled when their supply of liquid has been exhausted. This creates the possibility for spilling liquid during the refilling process. Additionally, the surface of the contact medium may deteriorate to the point that its application and cleaning functions are impaired. In this case, the contact medium must also be replaced, which generally requires an entire replacement drum maintenance cassette.
What is needed is a replaceable liquid intermediate transfer surface application system that overcomes the drawbacks of the prior art. The replaceable application system should be mechanically simple, have a low manufacturing cost and complexity and incorporate a minimum number of components. The system should eliminate any "time-to-first-print" delay and reliably deliver a precise amount of liquid to the intermediate transfer support surface. It is also desirable that this system include a self-contained and easily replaceable contact medium and liquid supply that may be conveniently removed and replaced by an operator without replacing the entire maintenance cassette to thereby reduce waste. The contact medium liquid/supply should also reliably contain the liquid and eliminate any risk of leak or spill, regardless of cassette orientation.